The GOP’s share of voters in Riverside and San Bernardino counties has waned in the last decade. New political boundaries, drawn last year, have let pent-up Democratic power push to the surface.

By Phil Willon, Los Angeles Times November 10, 2012, 7:50 p.m.

Stirred by a decade of astronomical growth, economic heartache and the rising political influence of Latinos, the Inland Empire proved treacherous territory last week for a Republican Party that just a decade ago considered it the new GOP frontier.

Voters in Riverside and San Bernardino counties on Tuesday elected three Democrats to Congress — two Latinos and a gay Asian American — after having sent only two Democrats to Washington in the last four decades.

Before the election, Republicans represented the city of Riverside in Congress, the state Senate and the Assembly. On Tuesday, Democrats took all three seats.

The rumblings of an impending seismic shift in Inland Empire politics have been heard for years, with pressure slowly building as the GOP’s share of voters declined. California’s new political boundaries, crafted last year, allowed pent-up Democratic power to push to the surface and reshape a political landscape that’s now more evenly divided. Contests will be much harder to predict.

“The Inland Empire was the third bastion for the GOP after Orange County and San Diego,” said Shaun Bowler, a political scientist from UC Riverside. “That’s not true anymore, which is a worry for the Republican Party. They’ve got to work harder than they have in the past.”

Tuesday’s election results showed that the GOP is far from dead, however, especially in areas such as Temecula, Corona and the high desert. And a conservative streak still runs through the region’s electorate.

Gov. Jerry Brown’s $6-billion-a-year tax initiative, Proposition 30, won statewide but lost in both Riverside and San Bernardino counties. Republican presidential nominee Mitt Romney won Riverside County. San Bernardino voters sided with President Obama, but by only 5 percentage points, compared with Obama’s 20-point victory statewide.

In 2010, voters in both counties favored Republican Meg Whitman over Brown in the gubernatorial race and Republican Carly Fiorina over Sen. Barbara Boxer (D-Calif.) for the U.S. Senate. Most of the Inland Empire still is represented by Republicans in Congress.

“It’s certainly more conservative than Marin County or Los Angeles, but demographic changes are killing Republicans,” said Jim Brulte of Rancho Cucamonga, a former leader of the state Senate’s Republicans, who is now a consultant. “The trend you’re seeing in the Inland Empire is no different than the trends everywhere in the state.”

A decade ago, a plurality of registered voters in both Riverside and San Bernardino counties were Republican. Democrats now claim a slim plurality in San Bernardino County. In Riverside County, the GOP’s 13-point registration advantage has dwindled to 4.5 points. The share of independent voters in both counties has soared.